In a typical radio communications network, wireless terminals, also known as mobile stations, terminals and/or user equipments, UEs, communicate via a Radio Access Network, RAN, to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g. a radio base station, RBS, which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. One base station may have one or more cells. A cell may be downlink and/or uplink cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
A Universal Mobile Telecommunications System, UMTS, is a third generation mobile communication system, which evolved from the second generation, 2G, Global System for Mobile Communications, GSM. The UMTS terrestrial radio access network, UTRAN, is essentially a RAN using wideband code division multiple access, WCDMA, and/or High Speed Packet Access, HSPA, for user equipments. In a forum known as the Third Generation Partnership Project, 3GPP, telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller, RNC, or a base station controller, BSC, which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System, EPS, have been completed within the 3rd Generation Partnership Project, 3GPP, and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network, E-UTRAN, also known as the Long Term Evolution, LTE, radio access, and the Evolved Packet Core, EPC, also known as System Architecture Evolution, SAE, core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network, RAN, of an EPS has an essentially “flat” architecture comprising radio base station nodes without reporting to RNCs.
LTE uses OFDM in the downlink and DFT-spread OFDM in the uplink. The basic LTE physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1. FIG. 1 shows an example of a LTE downlink physical resource.
Here, each resource element corresponds to one subcarrier during one OFDM symbol interval, i.e. an interval on a particular antenna port. An antenna port is defined such that the channel over which a symbol on the antenna port is conveyed may be inferred from the channel over which another symbol on the same antenna port is conveyed. There is one resource grid per antenna port.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of 1 ms as illustrated in FIG. 2. FIG. 2 shows an example of a LTE time-domain structure. A subframe is divided into two slots, each of 0.5 ms time duration.
The resource allocation in LTE is described in terms of resource blocks, where a resource block corresponds to one slot in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. Two in time consecutive resource blocks represent a resource block pair and corresponds to the time interval upon which scheduling operates.
Transmissions in LTE are dynamically scheduled in each subframe where the base station transmits downlink assignments/uplink grants to certain UEs via the physical downlink control channel or the enhanced physical downlink control channel (PDCCH and EPDCCH). The PDCCHs are transmitted in the first OFDM symbol(s) in each subframe and spans (more or less) the whole system bandwidth. A UE that has decoded a downlink assignment, carried by a PDCCH, knows which resource elements in the subframe that contain data aimed for the UE. Similarly, upon receiving an uplink grant, the UE knows which time/frequency resources it should transmit upon in uplink. In LTE downlink, data is carried by the physical downlink shared data link (PDSCH) and in the uplink the corresponding link is referred to as the physical uplink shared link (PUSCH).
The use of and enhanced downlink control signaling (EPDCCH) is available for terminals of Release 11 or later. Such control signaling has similar functionalities as PDCCH; with the fundamental differences of requiring UE specific DMRS instead of CRS for its demodulation and that the EPDCCH does only use a fraction of the system bandwidth which allows for interference coordination with other cells. One additional advantage is that UE specific spatial processing such as beamforming may be exploited for EPDCCH.
Demodulation of sent data requires estimation of the radio channel which is done by using transmitted reference symbols, RS, i.e. symbols known by the receiver. In LTE, cell specific reference symbols, CRS, are transmitted in all downlink subframes and in addition to assist downlink channel estimation they are also used for mobility measurements performed by the UEs. LTE also supports UE specific RS aimed only for assisting channel estimation for demodulation purposes.
FIG. 3 illustrates how the mapping of physical control/data channels and signals can be done on resource elements within a downlink subframe. More specifically, FIG. 1 shows an example of a mapping of LTE physical control signalling, data link and cell specific reference signals within a downlink subframe. In this example, the PDCCHs occupy the first out of three possible OFDM symbols, so in this particular case the mapping of data could start already at the second OFDM symbol. Since the CRS is common to all UEs in the cell, the transmission of CRS cannot be easily adapted to suit the needs of a particular UE. This is in contrast to UE specific RS which means that each UE has RS of its own placed in the data region of part of PDSCH or EPDCCH.
The length of the control region, which can vary on subframe basis, is conveyed in the Physical Control Format Indicator, PCFICH. The PCFICH is transmitted within control region, at locations known by terminals. After a terminal has decoded the PCFICH, it thus knows the size of the control region and in which OFDM symbol the data transmission starts. Also transmitted in the control region is the Physical Hybrid-ARQ Indicator, which carries ACK/NACK responses to a terminal to inform if the uplink data transmission in a previous subframe was successfully decoded by the base station or not.
As previously indicated, CRS are not the only reference symbols available in LTE. As of LTE Release-10, a new RS concept was introduced with separate UE specific RS for demodulation of PDSCH and EPDCCH and RS for measuring the channel for the purpose of channel state information (CSI) feedback from the UE. The latter is referred to as CSI-RS. CSI-RS are not transmitted in every subframe and they are generally sparser in time and frequency than RS used for demodulation. CSI-RS transmissions may occur every 5th, 10th, 20th, 40th, or 80th subframe according to an RRC configured periodicity parameter and an RRC configured subframe offset.
Time Division Duplex
Transmission and reception from a node, e.g. a terminal in a cellular system such as LTE, can be multiplexed in the frequency domain or in the time domain, or combinations thereof.
FIG. 4 shows an illustration of frequency- and time-division duplex. Frequency Division Duplex, FDD, as illustrated to the left in FIG. 4, implies that downlink and uplink transmission take place in different, sufficiently separated, frequency bands. Time Division Duplex, TDD, as illustrated to the right in FIG. 4, implies that downlink and uplink transmission take place in different, non-overlapping time slots. Thus, TDD can operate in unpaired spectrum, whereas FDD requires paired spectrum.
Typically, the structure of the transmitted signal in a communication system is organized in the form of a frame structure. For example, LTE uses ten equally-sized subframes of length 1 ms per radio frame as illustrated in FIG. 5. FIG. 5 shows an example of an uplink/downlink time/frequency structure for LTE in case of FDD and TDD.
In case of FDD operation, i.e. upper part of FIG. 5, there are two carrier frequencies, one for uplink transmission, fUL, and one for downlink transmission, fDL. At least with respect to the terminal in a cellular communication system, FDD may be either full duplex or half duplex. In the full duplex case, a terminal may transmit and receive simultaneously, while in half-duplex operation, the terminal may not transmit and receive simultaneously. However, in the latter case, the base station is capable of simultaneous reception/transmission though, e.g. receiving from one terminal while simultaneously transmitting to another terminal. In LTE, a half-duplex terminal is monitoring/receiving in the downlink except when explicitly being instructed to transmit in a certain subframe.
In case of TDD operation, i.e. lower part of FIG. 5, there is only a single carrier frequency and uplink and downlink transmissions are always separated in time also on a cell basis. As the same carrier frequency is used for uplink and downlink transmission, both the base station and the mobile terminals need to switch from transmission to reception and vice versa. According to an aspect of any TDD system is to provide the possibility for a sufficiently large guard time where neither downlink nor uplink transmissions occur. This is required to avoid interference between uplink and downlink transmissions. For LTE, this guard time is provided by special subframes, e.g. subframe 1 and, in some cases, subframe 6, which are split into three parts: a downlink part, DwPTS, a guard period, GP, and an uplink part, UpPTS. The remaining subframes are either allocated to uplink or downlink transmission.
TDD allows for different asymmetries in terms of the amount of resources allocated for uplink and downlink transmission, respectively, by means of different downlink/uplink configurations. In LTE, there are seven different configurations as shown in FIG. 6. FIG. 6 shows an example of different downlink/uplink configurations in case of TDD. Note that in the description below, DL subframe can mean either DL or the special subframe.
Currently there are 9 special subframe configurations defined for normal CP and 7 defined for extended CP, with different length of downlink pilot time slot, DwPTS, Guard Period, GP, and uplink pilot time slot, UpPTS. For normal CP, EPDCCH and PDSCH transmission is not supported for DwPTS spanning 3 OFDM symbols, i.e. configuration 0 and configuration 5. EPDCCH and PDSCH transmission is supported for all the remaining configurations with DwPTS spanning 9˜11 OFDM symbols.
To avoid severe interference between downlink and uplink transmissions between different cells, neighbor cells should have the same downlink/uplink configuration. If this is not done, uplink transmission in one cell may interfere with downlink transmission in the neighboring cell, and vice versa, as illustrated in FIG. 7. FIG. 7 shows an example of downlink/uplink interference in TDD. Hence, the downlink/uplink asymmetry can typically not vary between cells, but is signaled as part of the system information and remains fixed for a long period of time.
Enhanced Control Signaling in LTE
Messages transmitted over the radio link to users can be broadly classified as control messages or data messages. Control messages are used to facilitate the proper operation of the system as well as proper operation of each UE within the system. Control messages could include commands to control functions such as the transmitted power from a UE, signaling of RBs within which the data is to be received by the UE or transmitted from the UE and so on.
In Rel-8, the first one to four OFDM symbols, depending on the configuration, in a subframe is reserved to contain such control information, see e.g. FIG. 3. Furthermore, in Rel-11, an enhanced control channel was introduced, EPDCCH, in which PRB pairs are reserved to exclusively contain EPDCCH transmissions, although excluding from the PRB pair the one to four first symbols that may contain control information to UEs of releases earlier than Rel-11. This is illustrated in FIG. 8.
In FIG. 8, the downlink subframe shows 10 RB pairs and configuration of three EPDCCH regions, i.e. red, green, and blue, of size 1 PRB pair each. The remaining PRB pairs may be used for PDSCH transmissions.
Hence, the EPDCCH is frequency multiplexed with PDSCH transmissions contrary to PDCCH which is time multiplexed with PDSCH transmissions. Note also that multiplexing of PDSCH and any EPDCCH transmission within a PRB pair is not supported in LTE Rel-11. A UE may be configured to monitor the EPDCCH in one or two sets of RB pairs and the RB belonging to each set is independently configured by RRC signaling.
The PDCCHs and EPDCCHs are transmitted over radio resources that are shared between several UEs. Each PDCCH consists of smaller parts, known as control channel elements, CCEs, to enable link adaptation, e.g. by controlling the number of CCE a PDCCH is utilizing. It is specified that for PDCCH, a UE has to monitor 4 aggregation levels of CCEs, namely, 1, 2, 4, and 8, for UE-specific search space and 2 aggregation levels of CCEs, namely, 4 and 8, for common search space.
According to one example, a search space Sk(L) at aggregation level L∈{1,2,4,8} is defined by a contiguous set of CCEs given by:(Zk(L)+i)mod NCCE,k 
where NCCE,k is the total number of CCEs in the control region of subframe k,
Zk(L) defines the start of the search space, i=0, 1, . . . , M(L)·L−1, and
M(L) is the number of PDCCHs to monitor in the given search space. Each CCE contains 36 QPSK modulation symbols.
Here, it may be noted that NCCE,k is dependent on the subframe index k. This because the number of control OFDM symbols, i.e. 1, 2, 3 or 4, may change from subframe to subframe. In addition, the number of physical HARQ indicator channels, PHICH, may also change from subframe to subframe in TDD.
The value of M(L), for example, be specified by Table 1, as shown below:
TABLE 1M(L) vs. Aggregation Level L for PDCCHSearch space Sk(L)Number of Aggregation levelSize PDCCH candidatesTypeL[in CCEs]M(L)UE-specific16621264828162Common41648162
With this definition, search space for different aggregation levels may overlap with each other regardless of system bandwidth. More specifically, UE-specific search space and common search space might overlap and the search spaces for different aggregation levels might overlap.
One example is shown below in Table 2 where there are 9 CCEs in total and very frequent overlap between PDCCH candidates:
TABLE 2NCCE,k = 9, Zk(L) = {1, 6, 4, 0} for L = {1, 2, 4, 8}, respectively.Search space Sk(L)AggregationTypeLevel LPDCCH candidates in terms of CCE indexUE-1{1}, {2}, {3}, {4}, {5}, {6}Specific2{6, 7}, {8, 0}, {1, 2}, {3, 4}, {5, 6}, {7, 8}4{4, 5, 6, 7}, {8, 0, 1, 2}8{0, 1, 2, 3, 4, 5, 6, 7}, {8, 0, 1, 2, 3, 4, 5, 6}Common4{0, 1, 2, 3}, {4, 5, 6, 7}, {8, 0, 1, 2}, {3, 4, 5, 6}8{0, 1, 2, 3, 4, 5, 6, 7}, {8, 0, 1, 2, 3, 4, 5, 6}
Similar as for PDCCH, the EPDCCH is transmitted over radio resources shared by multiple UEs and enhanced CCE (ECCE) is introduced as the equivalent to CCE for PDCCH. An ECCE has also a fixed number of RE but the number of RE available for EPDCCH mapping is generally fewer than this fixed number because many RE are occupied by other signals such as CRS and in some subframes CSI-RS. Code chain rate matching is applied whenever a RE belonging to a ECCE contains other colliding signals such as the CRS, CSI-RS, legacy control region or in case of TDD, the GP and UpPTS.
Note also in this case that the number of ECCE per EPDCCH set p, denoted NECCE,p,k is dependent on the subframe index k. This is due to the fact that under some circumstances, as in TDD special subframes and/or in subframes with extended CP, the number of ECCEs is generally fewer.
In addition, the search space for EPDCCH, denoted ESk(L) has a larger set of aggregation levels than PDCCH, namely L∈{1,2,4,8,16,32}, see section 9.1.4 in 3GPP TS 36.213. But, in a given subframe not all six aggregation levels are available, at most five are, and commonly only four. So the set of aggregation levels also varies from subframe to subframe, depending on the varying overhead of other signals such as CSI-RS and the legacy control region size, i.e. 1, 2, 3 or 4 OFDM symbols, comprising PDCCH, PCFICH and PHICH.
In Rel-11, the EPDCCH supports only the UE specific search space whereas the common search space remains to be monitored in the PDCCH in the same subframe. In future releases, the common search space may be introduced also for EPDCCH transmission.
As mention above, it is specified that the UE monitors ECCE aggregation levels 1, 2, 4, 8, 16 and 32 with restrictions, shown in Table 4, where nEPDCCH is the number of available RE for EPDCCH transmission in a PRB pair. In Table 4, distributed and localized transmission refers to the EPDCCH mapping to resource elements.
In distributed transmission, an EPDCCH is mapped to resource elements in up to D PRB pairs, where D=2, 4, or 8. However, it may be noted that the value of D=16 is also being considered in 3GPP. In this way can frequency diversity be achieved for the EPDCCH message.
FIG. 9 shows a schematic example wherein downlink subframe showing 4 parts belonging to an EPDCCH is mapped to multiple of the enhanced control regions known as PRB pairs, to achieve distributed transmission and frequency diversity or subband precoding.
In localized transmission, an EPDCCH is mapped to one PRB pair only, if the space allows, which e.g. is always possible for aggregation level one and two and for normal subframes and normal CP length also for level four. In case the aggregation level of the EPDCCH is too large, a second PRB pair is used as well, and so on, using more PRB pairs, until all ECCE belonging to the EPDCCH has been mapped. The number ECCE that fit into one PRB pair is given by Table 3 below:
TABLE 3Normal cyclic prefix SpecialSpecial subframe,subframe,Extended cyclic prefixconfigu-configu-Special subframe,Normalration ration Normalconfiguration subframe3, 4, 81, 2, 6, 7, 9subframe1, 2, 3, 5, 644222
Table 3 shows the number ECCE that fit into one PRB pair, i.e. the number of ECCE per PRB pair. Note that special subframe 0 and 5 for normal CP and 0 and 4 for extended CP are missing from the table, since in these subframes EPDCCH is not supported at all, i.e. zero ECCEs.
FIG. 10 shows an illustration of localized transmission. More specifically, FIG. 10 illustrates a downlink subframe showing the 4 ECCEs belonging to an EPDCCH is mapped to one of the enhanced control regions, to achieve localized transmission.
As an example, in normal subframe and with normal CP length and with nEPDCCH≥104, localized transmission is using aggregation levels (1,2,4,8) and they are mapped to (1,1,1,2) PRB pairs respectively.
To facilitate the mapping of ECCEs to physical resources each PRB pair is divided into 16 enhanced resource element groups, EREGs, and each ECCE is split into 4 or 8 eREGs for normal and extended cyclic prefix, respectively. An EPDCCH is consequently mapped to a multiple of four or eight EREGs depending on the aggregation level. These EREG belonging to an EPDCCH resides in either a single PRB pair, as is typical for localized transmission, or a multiple of PRB pairs, as is typical for distributed transmission.
One example of an exact division of a PRB pair with normal CP and normal subframe into EREG is illustrated in FIG. 11. FIG. 11 shows a PRB pair of normal cyclic prefix configuration in a normal subframe. Each tile is a resource element where the number corresponds to the eREG it is grouped within. The dashed areas correspond to the first eREG indexed with 0. Furthermore, it is specified in 3GPP TS 36.211 how the 4 or 8 EREGs respectively are grouped into the ECCEs. Table 4 shows aggregation levels for EPDCCH.
TABLE 4Aggregation levelsNormal subframes and specialsubframes, configuration 3, 4, 8,with nEPDCCH < 104 and usingnormal cyclic prefixAll other casesEPDCCH LocalizedDistributedLocalizedDistributedformattransmissiontransmissiontransmissiontransmission02211144222884431616884—32—16Mapping of EPDCCH to RE
Each EPDCCH comprises of AL ECCEs, where AL is the aggregation level of the message. Each ECCE in turn consists of L EREG, where L=4 or L=8. An EREG is a group of RE which are defined in 3GPP specification TS 36.211. In each PRB pair there are 16 EREG.
When EPDCCH collides in mapping with other signals such as own cell CRS or own cell legacy control region, the other signals have priority and EPDCCH is mapped around these occupied REs and code chain rate matching is applied. This means that the effective number of available RE per EREG is usually less than the 9 RE but there is no interference from these colliding signals introduced in the decoding since the EPDCCH is mapped around those.
Work is ongoing in 3GPP to enhance the coverage for machine type communication (MTC) devices, a special category of UEs, and to achieve in the order of 15-20 dB coverage enhancements in LTE multiple physical channels and physical signals will need to be improved. Since the required improvements are so large, i.e. 20 dB coverage improvements is equivalent to operation at 100 times lower signal-to-noise ratio, and LTE is already very good, i.e. there is no known flaw in LTE that can provide improvements anywhere near 100 times, it is likely that plain old repetition will provide most of the required coverage improvements. However, current LTE signals cannot easily just be repeated approximately 100 times, for example, due to timing constraints during connection setup and other procedures, so new signals may need to be defined for this purpose.
According to the above, there is a need to improve the radio coverage of a control channel when using repeated control channel transmissions in a radio communications network.
In a document R1-135461 entitled—“PDCCH transmission for MTC coverage enhancement”, a method for PDCCH coverage enhancement is outlined, where a PDCCH transmission is repeated, beginning at a start subframe and comprising a number of subsequent subframes. One simple option is to use same PDCCH candidate index during a PDCCH repetition. Then, an UE may assume that PDCCH candidate index of repeated PDCCHs are same with that of first PDCCH in a bundle. It allows the same UE blind decoding complexity and makes simple UE behavior, and additional signaling is not required.